Pumps of this type can be used notably for the in situ administration of medicaments whereby miniaturization of the pump enables the patient to wear it on his person or possibly to have a pump directly implanted in his body. In addition, using pumps of this type, small quantities of liquid to be injected can be accurately administrated.
In an article entitled "A piezoelectric micropump based on micromachining of silicon" published in "Sensors and Actuators" No. 15 (1988), pages 153 to 167, H. van Lintel et al describe two embodiments of a micropump each comprising a stack of three wafers, i.e. a wafer of machined silicon arranged between two wafers of glass.
The silicon wafer defines a pump chamber with one of the glass wafers, it being possible for the part coinciding with this chamber to be deformed by drive means, in the present case a piezoelectric crystal. The latter comprises electrodes which, when connected to an alternating voltage source, cause deformation of the crystal and thus of the glass wafer, the latter in turn causing variation in the volume of the pump chamber.
The pump chamber is connected on each side to check valves machined from silicon the seat of which is composed of the other glass wafer.
The mode of operation of this type of pump is influenced by the compressibility of the fluid and it may consequently not work if it contains too much air.
Various techniques exist for priming these micro-pumps.
According to a first technique, priming occurs under vacuum. The inlet of the micropump is linked to a fluid reservoir, for example to a syringe, and the outlet of the micropump is linked to a vacuum pump : starting up the latter causes aspiration of the air of the micropump and injection of fluid. A second technique consists of using an enclosure connected to a vacuum pump and partially filled with fluid. After plunging the micro-pump in the fluid, the vacuum pump is activated to remove air from the enclosure and then the pressure in the enclosure is increased by re-introducing air, thereby forcing the fluid into the micropump. This operation is repeated several times to ensure that sufficient air has been expelled from the micropump. It will be understood that these two techniques need a considerable amount of equipment and that they cannot therefore be carried out in a doctor's surgery, but only in a hospital environment or in a factory, during manufacture.
The micropump may also be primed under normal atmospheric pressure. Fluid is simply injected into the micropump using a syringe by means of successive strockes or continuously. Air is normally driven by the fluid or carried therewith towards the outlet of the micropump. Nevertheless, due to the shape of the pump chamber and of the channels of the micropump, air bubbles could form and remain trapped in the micropump. This risk can be reduced by increasing the pressure at which the fluid is injected. Nevertheless the elimination of the air bubbles remains largely unreliable.